1. Field of the Invention
The present invention relates to automatic speed change apparatus wherein drive paths connected to a plurality of elements of planetary gear sets are controlled by hydraulic clutches and brakes and, more particularly, to a front-wheel-drive automatic speed change apparatus comprising an input shaft, an intermediate shaft and an output shaft which are arranged in parallel, with a differential gear provided on the output shaft.
2. Description of the Related Art
Four-forward-speed, one-reverse-speed speed change apparatuses are known which have incorporated therein a torque converter having a lockup clutch as passenger motor vehicle automatic transmissions which are most prevalently used. Front-wheel-drive four-forward-speed, one-reverse-speed automatic speed change apparatuses generally comprise a first shaft coaxial with an engine and provided via a torque converter with three clutches, two brakes, two planetary gear sets and a one-way clutch, and further comprise a transmission having an intermediate shaft and an output shaft, such that power is transmitted via counter gears and the second shaft, i.e., the intermediate shaft, to the third shaft serving as the output shaft.
The gear ratio of the speed change apparatus is dependent on the maximum traction force and the maximum speed of the motor vehicle, and the greater the number of speeds, the more advantageous the traction characteristics and fuel consumption. However, the apparatus then has the drawbacks of becoming more complex, lower in transmission efficiency, more costly and greater in axial length. For this reason, increases in the number of speeds available for front-wheel-drive automatic speed change apparatus are from three forward speeds, one reverse speed to four forward speeds, one reverse speed, namely only to such a number of speeds which can be achieved by additionally using a single clutch only without entailing a substantial increase in the axial length.
Conventionally used for giving three forward speeds and one reverse speed is an arrangement of two planetary gear sets which is small in the load on the gear tooth faces and highly efficient and in which a sun gear is coupled to a sun gear, with a planetary carrier connected to a ring gear, to give the input to the ring gear for forward speeds. Nevertheless, the planetary gear set wherein the input is given to a sun gear, and the load on the gear tooth faces is great because of a small diameter is used in the four-forward-Speed, one-reverse-speed automatic speed change apparatus in place of the above arrangement because the single drive path to be additionally installed in the apparatus will interfere with the above gear train arrangement.
The apparatus requires only one additional clutch, whereas if the elements of the apparatus are arranged concentrically on the first shaft, this imposes a limitation on the shortening of the axial length of the apparatus partly because of the additional provision of the single drive path.
Although the four-forward-speed, one-reverse-speed automatic speed change apparatus is an improvement over three-forward-speed, one-reverse-speed automatic speed change apparatus, the overall gear ratio range obtained by dividing the gear ratio of first speed by the gear ratio of the fourth speed is as small as about 4, while the individual gear steps need to be great, so that the use of the engine at low speeds and the use of lockup of the torque converter for an improved fuel consumption still remain insufficient.
As is well known, environmental problems of the earth have led to strong demands for lower automotive fuel consumptions in recent years, and change-over from gasoline engines to efficient diesel engines appears feasible. Diesel engines are lower than gasoline engines in maximum number of revolutions. Accordingly, it is desired more than ever that an automatic speed change apparatus be adapted to give six forward speeds which involve an overall gear ratio range of at least 5 in order to compensate for the drawback of four-forward-speed, one-reverse-speed devices.
On the other hand, strong demands are also made for higher safety from the collision of vehicles, with the result that the space laterally available in the engine room has become ever smaller due to the provision of reliable side members. Thus, it is desired that the speed change apparatus be further reduced in axial length. Because side members have a larger width when positioned closer to the driver's seat from the viewpoint of strength, it is desirable to provide a shorter length to the intermediate shaft, i.e., the second shaft, which is positioned close to the driver's seat than to the input shaft, i.e., the first shaft.
To overcome these problems, JP-A No. 4-219553 (U.S. Pat. No. 5,106,352 EP 434525), JP-A No. 2001-221301 and U.S. Pat. No. 5,013,289 propose six-forward-speed, one-reverse-speed automatic speed change apparatuses of low cost and high efficiency which comprise two planetary gear sets having four elements, a plurality of counter gear sets which are different in gear ratio, three clutches and two brakes.
JP-A No. 4-219553 discloses a front-wheel-drive speed change apparatus for use with an engine having an elongated shaft. The apparatus has a first shaft provided with a torque converter only, and an output shaft greatly offset to avoid interference with the engine, and an elongated output shaft provided with clutches, planetary gear train and a differential gear. Thus, the apparatus differs from the apparatus of the present invention.
JP-A No. 2001-221301 discloses the combination of the gear ratios of counter gear sets and planetary gear sets, with reference to an embodiment. However, this combination is not applicable to the planetary gear sets for use in the present invention wherein an input is given to the ring gear to obtain first to fourth forward speeds.
These gear train comprise a plurality of counter gears which are subjected to a great radial load, and the radial bearings for the counter gears therefore need to have a high strength, whereas the schematic drawing referred to in the embodiment shows that the bearings for the counter gears of the planetary gear set on the second shaft are provided unsuitably, namely, not provided directly on the second shaft.
U.S. Pat. No. 5,013,289 discloses planetary gear sets which are similar to those of JP-A No. 2001-221301. However, these gear trains are not applicable to the planetary gear sets for use in the present invention wherein an input is provided to the ring gear to obtain first to fourth forward speeds. The disclosed apparatus further has the drawback of being elongated in the axial direction because the bearings for counter gears are retained on a housing and also because three clutches are arranged on a first shaft serving as the input shaft. Additionally, because the counter gear is mounted on a shaft end, the second shaft has the same length as the first shaft. Further in the case where the bearings for the counter gears are held by the housing which is at rest, this mode of supporting the bearings is disadvantageous with respect to efficiency, durability and noise because the bearings are rotating at all times.
With the six-forward-speed, one-reverse-speed automatic speed change apparatus heretofore in use, the parking gear is disposed on the second shaft having a large space and therefore poses no problem, whereas according to U.S. Pat. No. 5,013,289 and JP-A No. 2001-221301, the planetary gear sets are arranged on the second shaft, which therefore has the drawback of becoming elongated when provided with the parking gear.